1. Field of the Invention
This invention relates to a dielectric resonator for high frequencies, used in communication machinery, and also relates to a process for its manufacture.
2. Description of the Prior Art
As a material for conductive films of dielectric resonators for high frequencies, silver has been hitherto commonly used. In the formation of a conductive film comprised of silver, silver baking has been used. For this purpose, a silver paste comprising a mixture of at least silver and glass frit is adhered to a dielectric ceramic by a coating means such as brushing, followed by heating to bake silver metal on the surface of the dielectric ceramic so that a conductive film is formed. Hence, although the silver has originally an electrical conductivity of 6.06.times.10.sup.5 (1/.OMEGA..cm), the silver conductive film formed by this baking results in a low electrical conductivity which is about 20% of the original conductivity, because of the presence of the glass frit mixed. Since, however, this glass frit is added for the purpose of obtaining an adhesion strength between the dielectric ceramic for high frequencies and the silver metal, an attempt to form a conductive film using a silver paste not containing this glass frit brings about a serious lowering of the adhesion strength between the conductive film and the dielectric ceramic, making it impossible to use the film as a conductive film. Since also the silver is an expensive metal, there is an anxiety for a high manufacture cost.
Under such circumstances, it is attempted to use inexpensive copper as a material for the conductive film in place of the expensive silver. In the case when copper conductive films are formed, they are usually formed by plating. However, the conductive film having been formed by plating without any additional treatment has so low an electrical conductivity that the value Q of a dielectric resonator comprising such a conductive film formed becomes smaller. Hence, after the conductive film has been formed on the ceramic, the conductive film must be subjected to a heat treatment carried out in an environment of inert gas such as nitrogen or argon so that the electrical conductivity of the conductive film can be increased. The reason why the heating is carried out in an environment of inert gas is to prevent the conductive film from being oxidized to make solderability poor or its contact resistance from becoming greater.
Since, however, the formation of conductive films by such a method requires the heating carried out in an inert gas environment, this method has been involved in the problem that the manufacturing process is complicated to bring about a poor productivity. It has been also involved in the problem that the conductive films formed of copper usually tend to become corroded.